This invention relates to the preparation of sodium carbonate through wet calcination of sodium bicarbonate or sodium sesquicarbonate by contact with steam and, further, to the conversion of a wet product into dense soda ash of a practically uniform particle size.
Sodium carbonate, immensely consumed in many industries, is obtained mostly by the pyrolysis of sodium bicarbonate or sodium sesquicarbonate prepared by the ammonia soda process or ammonium chloride soda process. In the ammonia soda process, carbon dioxide gas is passed through an ammonia-saturated salt-water to precipitate sodium bicarbonate, followed by separation of the precipitate and washing. This crude sodium bicarbonate is calcined to give sodium carbonate as represented by the following equation. EQU 2NaHCO.sub.3 +heat.fwdarw.Na.sub.2 CO.sub.3 +CO.sub.2 .uparw.+H.sub.2 O (1)
In soda plants where the ammonia soda process is employed, it is a usual practice to prepare sodium hydroxide by adding lime milk to a sodium carbonate solution to accomplish causticization according to the following equation. EQU Na.sub.2 CO.sub.3 +Ca(OH).sub.2 .fwdarw.2NaOH+CaCO.sub.3 ( 2)
For this purpose, a so-called "wet calcination" process for the preparation of a sodium carbonate solution is more prevailing than the above described dry calcination process. In the wet calcination process, crude sodium bicarbonate is poured into water with stirring to give a suspension which is about 580 g/l in concentration of NaHCO.sub.3, and the suspension is allowed to make a countercurrent contact with a slightly pressurized steam in a decomposition tower of the type having bubble-cap plates or perforated plates in a multi-deck arrangement to achieve an efficient gas-liquid contact. Through pyrolysis in the decomposition tower, the bicarbonate suspension turns into a relatively thick sodium carbonate solution in which the Na.sub.2 CO.sub.3 concentration is about 300 g/l.
The decomposition of sodium bicarbonate in this process, however, is always incomplete: the decomposition remains only 85-87% at the highest. As a consequence, the above described causticization of the resultant sodium carbonate solution (hereinafter soad solution) requires a considerable excess of lime milk, meaning a wasteful contribution to the cost of caustic soda, since the bicarbonate remaining undecomposed in the soda solution consumes twice as much equivalence of calcium hydroxide than the carbonate.
In the case of providing sodium carbonate anhydride as a commercial chemical, commonly under the name of soda ash, the anhydride must take the form of crystalline particles of a relatively high bulk density such as 1.05-1.2 g/cm.sup.3 (so-called dense ash) having uniform particle size. The calcination of crude sodium bicarbonate obtained in the ammonia soda process gives sodium carbonate anhydride particles of a relatively low bulk density, that is, a so-called light ash. The light ash is sufficiently moistened with water and then kneaded to form crystalline sodium carbonate monohydrate. The calcination of the crystalline monohydrate gives the anhydride as crystalline particles with a bulk density worthy of a dense ash. As a disadvantage of this method, it is difficult to obtain a dense ash having a satisfactorily uniform particle size. One must regulate the particle size of the calcined product by sieving to obtain a commercially valuable soda ash.
When naturally occurring trona (chemically, sodium sesquicarbonate) is used as the material of soda ash, dense ash is obtained by first calcining trona, preparing a solution of the resultant sodium carbonate anhydride, removing impurities from the solution, concentrating the solution to cause sodium carbonate monohydrate to crystallize from the solution and calcining the crystalline monohydrate separated from the solution. In this case it is possible to obtain dense ash of a satisfactorily uniform particle size by adopting some measures to control the crystallization of the monohydrate from the concentrated solution. However, only a limited supply of soda ash comes from naturally occurring trona.
Referring again to the ammonia soda process, it has been tried to obtain sodium carbonate anhydride as dense ash from the above described soda solution through concentration for crystallization of sodium carbonate monohydrate and calcination of the monohydrate following the separation from the mother liquor and washing. In this case, however, the incomplete decomposition of sodium bicarbonate at the wet calcination offers a serious problem to the crystallization of the sodium carbonate monohydrate. It is inevitable that sodium sesquicarbonate crystallizes from the soda solution obtained in this process together with sodium carbonate, resulting in that soda ash given by the wet calcination is satisfactory neither in purity nor in bulk density. Besides, a large energy consumption is needful for the concentration of the soda solution. According to one of improvements hitherto proposed on this method, the soda solution is made to turn into a complete sodium carbonate solution by the addition of caustic soda prior to the concentration of the solution, but this improvement does not solve the problem of large energy consumption for the concentration since the soda solution obtained by the wet calcination process is always an undersaturated solution. Due to such an uneconomical nature, use has not been made of the soda solution obtained by the wet calcination process in the industrial preparation of soda ash.